This invention relates to testing of moving-coil type geophones to determine their operability.
A moving-coil geophone comprises a coil of wire suspended in a magnetic field by means of springs. The magnetic field is produced by a permanent magnet which is attached to the case of the geophone. When the geophone is secured to the ground, seismic vibrations cause relative motion between the magnetic field and the coil so as to induce a voltage in the coil. This voltage can be measured to derive conventional seismic traces.
A geophone can be rendered totally or at least partially inoperable in several ways. For example, breakage of the spring mechanism causes total inoperability. Or, a geophone can accidentally fall from its normal vertical position in the field so as to lay on its side in a horizontal position, thus not permitting proper movement of the coil. Geophone performance can also be affected by other factors such as defects in the coil or its associated connections, etc.
Therefore, it is highly desirable before a seismic test to test the operability of an array of geophones which will be used in the field. It is also desirable to test geophones in the laboratory before sending them out to the field so that repairs can conveniently be made.
Various methods for testing geophones have been devised. One common method is to measure instantaneous voltage at two particular points on a levitate waveform produced by the geophones, and also time at two additional points. A levitate waveform is produced by a geophone by applying thereto a current which causes displacement of the coil, and then removing that current to cause the coil to return to its original position. Movement of the coil back to its original position produces a voltage waveform which is commonly known as the levitate waveform. The above-mentioned measurements of amplitude and time are used to compute resonant frequency, damping, and transductance. This technique requires highly accurate measurement of voltage and time, is susceptible to both vibrational and electrical noise, and involves complex computations using natural logarithms or trigonometric functions.